STATE HOUSE-The details of how the MBTA would be compensated for a planned Olympic stadium above its existing facilities have not yet been determined, a point raised by an environmental advocate and acknowledged by Olympics backers.

The marquee venue, Olympic Stadium, would sit above tracks used by the MBTA’s Red Line and designs show the stadium infrastructure housing a T bus yard that is currently at the site.

The existing New Boston Food Market and a related cold-storage facility make up more than 21 acres of the stadium’s proposed location at Widett Circle, the city has close to 18 acres for public works usage, Amtrak has a piece and the T has about 23 acres for its bus facility, according to Boston 2024 bid documents. About 6.8 acres of development would occur over active rail, according to the bid document.

The $1.2 billion plan for prepping the infrastructure at the site Olympics backers dubbed Midtown sets out $125 million for acquisition of land for the stadium, more than $96 million for relocating bus maintenance to beneath the planned plaza and $10 million in air rights that would be paid to Amtrak along with other costs. Those costs are outlined in an updated “bid 2.0” submitted by the non-profit group hoping to bring the Summer Games to Massachusetts in 2024.

Rafael Mares, a senior attorney at the Conservation Law Foundation, expressed concerns to the News Service that the new bid sets aside no specific money for air rights for the T. Air rights are transferred to a developer allowing the group to build over something else, such as a highway or railways.

“Boston 2024 has reviewed the air rights issue and would only need to acquire rights for a relatively small section — and only to enhance pedestrian access not for future development,” Boston 2024 Chief Operating Officer Erin Murphy said in a statement to the News Service. “We are confident this can be accomplished and look forward to productive conversations with the state and the MBTA as we move forward on these exciting urban renewal plans for Widett Circle.”

A Boston 2024 official said the MBTA would be fully compensated through a land swap that would offer the T more room. The land swap would give the T space beneath the deck that would support the Olympic stadium, according to a Boston 2024 official, who noted the stadium would be converted into a public park.

The official also raised the possibility that the T could retain ownership of its parcel, benefitting from future development at the site.

“The MBTA can’t be expected to give away valuable property rights to a private developer without compensation. The transit agency is in no financial position to do so, nor should it since that would be inconsistent with Boston 2024’s oft repeated promise that no public resources will be used to pay for the Olympics,” Mares said in a statement. “Even if Boston 2024 expects the MBTA to give away its air rights, the value of such a contribution, in the interest of full transparency, should still be reflected in the budget. Bid 2.0 should be revised accordingly.”

An official at the Boston Redevelopment Authority said he is not aware of any rule-of-thumb about the dollar value of air rights. Recently permitted projects involving air rights are the Fenway Center project led by John Rosenthal, and a Back Bay development led by the state-designated Peebles Corporation, according to the BRA.

The MBTA is awaiting the analysis of The Brattle Group, tapped by the state to issue a report on the Olympics proposal. T spokesman Joe Pesaturo said, “The MBTA is pleased that Boston 2024 has put forward a far more extensive plan and we look forward to working with the experts Governor Baker has tapped to thoroughly examine that plan.”

Sitting between the South End and South Boston, the parcels of industrial land surrounded by highway and rail beds had “not really” been on developers’ radar before Boston 2024 proposed planting a stadium there, according to David Begelfer, CEO of NAIOP Massachusetts, which represents commercial developers.

While developers may have overlooked the site, they are now taking notice, said Begelfer. Asked if he himself had any financial interest in the property, he said, “I wish I did.”

The value of land depends greatly on what can be built there, said Begelfer, who said the closest comparison in scale to Widett Circle is Beacon Park Yard, a former freight yard in Allston now owned by Harvard.

Begelfer declined to offer his own estimates of what land or air rights would be worth at the circle, though he said the timetable for compiling the properties, sorting out zoning, laying the infrastructure and then building a stadium ready for the world stage nine years from now is doable.

“There’s no question that that can be accomplished in that period of time,” Begelfer said. He said a collaborative relationship with City Hall is important. Boston Mayor Marty Walsh lobbied for Boston to be selected as the nation’s bid city and has defended the proposal against opponents who argue it saps energy from more important projects and would put taxpayers on the hook for cost overruns.

The city has also not come to agreement on a price for its land, according to a City Hall spokeswoman.

Mares offered his own back-of-envelope calculation for what he said the air rights for the bus facility alone might be worth. Noting the roughly 25-acre bus facility alone is about 18 times larger than the 1.4 acres owned by Amtrak – which is set to receive $10 million in air rights, according to the bid – Mares said the T should receive something in the order of $180 million for the bus yard alone.

A Boston 2024 official disputed that figure, arguing the land swap would give the MBTA more valuable land.